Hello Cristina,
we have the same problem an i have found a way to find out the user who dropped the member:
From GUI:
Use "ChangePackage"->"Find"
Member type=member
Project=yourProject
Member=membername
Entry type=drop
Here you can find the ChangePackage which was used for the Drop-Command and also the user who created/used the ChangePackage.
From CMD:
si viewcps --filter=member:"membername" --filter=type:drop --filter=project:"yourprojectpath" --filter=membertype:member
This way works...but it would be nice if there is a more easier way to get the information for dropped members/subprojects.